Membership Upgrade buttons missing and how to give upgrade discount

I have setup 4 paid membership accounts, each giving more and more access to content on the website. My question is, how does one upgrade from one account to another? I have an "Update Accounts" page with the [renewform] shortcode on the page and it shows all the available accounts to upgrade to but with now way of upgrading, no buttons or links or anything.

Also, if there IS a way to upgrade is there a way to discount the price, like say they purchase a lower account for $10 and decide they need more access and upgrade to a $30 level, is there a way to automatically discount the next level by $10 or a certain percentage cause they've already paid?

Thanks! I am attaching a screenshot of the accounts page.